Cbache/creation of dict config getter

DictConfigGetter behave like ConfigGetter, except it can also be initialize with dictionary. We use this for testing purpose.
@dzen 
@rbarrois 
Since we don't want to modify the input, we might use a tuple since it's faster and more memory efficient.  source: http://stackoverflow.com/a/1708538
Therefore tuple are generally used for heterogeneous data. source : http://stackoverflow.com/a/626871
So, what is better ?